What's in an interior rearview mirror? You name it because it may be there! Because of its optimal location, the rearview mirror has become a centerpiece for modern day electronic devices. 

Figure 1. This moisture sensor, used for an automatic windshield wiper system, stems from the base of a rearview mirror.

Figure 2. A light sensitive sensor, used for an auto-dimming high-beam system, is located at the base of this rearview mirror.

Figure 3. Among the controls found on this rearview mirror is a button for OnStar.

Some of the new electronic accessories that are integrated with the interior rearview mirror include: automatic windshield wipers; auto-dimming high-beam systems; wireless connection devices; computer control modules; and auto-dimming rearview mirrors.

Automatic windshield wipers

An interior rearview mirror is a common location for moisture sensors used by automatic windshield wiper systems. The moisture sensor and control module may consist of a single assembly and be attached to the base of the rearview mirror (Figure 1), or only the sensor is placed there and the module is located somewhere else in the vehicle. The sensors project infrared beams onto the outer layer of the windshield. The presence and amount of moisture accumulated on the windshield is determined by how the beams reflect back into the sensor.

When the windshield is dry, most of the beams reflect back into the sensor. If there is moisture, the beams are interrupted and deflect in different directions. The more rain that is on the windshield, the fewer beams that make it back to the sensor. A special type of windshield may also be required for some systems.

Auto-dimming high beams

The new auto-dimming high-beam systems equipped on the 2005 Jeep Grand Cherokee and some of the new Cadillac vehicles also use a sensor that is located on the rearview mirror (Figure 2). The sensor for these systems is used to detect light projected from other vehicles. When the sensor detects light, either from oncoming headlamps or preceding tail lamps, the high beams automatically transition to the low beams.

The new system is much more advanced than older versions. The sensor is one type commonly used in digital cameras. It can distinguish between light produced from vehicles and light produced from street lights (DC vs. AC light). Older versions of this feature would often react to non-vehicle lights and flick on and off erratically. The new systems are also designed to transition more smoothly between high and low beams, but will instantly turn off if a vehicle suddenly appears.

Wireless connections

Wireless technology systems like OnStar and Bluetooth also use the convenient location of the rearview mirror for system controls and microphones (Figure 3). OnStar is a wireless service that provides a wide range of assistance to vehicle passengers. The OnStar button may be located on the rearview mirror, and is pressed to contact a service operator that can provide directions to the nearest hotels and restaurants or contact emergency personnel and provide them with the GPS location of the vehicle. This system may also automatically activate upon air bag deployment.

Figure 4. The electronic control module for the Volvo S60 is located inside the rearview mirror.

Bluetooth is a wireless communication system used to link mobile phones, computers, and other devices over short distances. These systems are being added to vehicle designs to provide hands-free mobile phone use using the driver's own mobile phone, provided that the mobile phone is Bluetooth compatible. Many of the newer factory installed systems are voice activated, with the microphone located on the rearview mirror, and broadcasts through the vehicle speakers.

Computer control modules

Late model Volvo vehicles use the interior rearview mirror for housing the upper electronic module (UEM) (Figure 4). This module controls the interior accessories of the vehicle including, but not limited to, receiving lock and unlock commands from the key fob. The UEM cannot be separated from the mirror and both will require replacement as a single unit if either is damaged. If the rearview mirror/UEM is replaced, special software is required during the installation process.

Auto-dimming rearview mirror

Not only can a rearview mirror house many electronic accessories, but it may actually be an electronic accessory itself. The function of an auto-dimming rearview mirror is to shade out headlamp glare from vehicles in the rear. Glare from tailing headlamps are sometimes so bright that they can temporarily blind a driver.

These mirrors use an electrochromic material sandwiched inside the glass of the mirror. Electrochromic is the combination of two words - "electro" referring to electric, and "chromic" referring to color. The material changes color, or darkens, when exposed to an electrical current and dims the reflection of the mirror.